1. Field of the Invention
This invention relates to a display apparatus suitable for use in an environment in which it is likely influenced by external light such as a display apparatus for a vehicle.
2. Description of the Prior Art
Various display apparatus for a vehicle are already known, and an exemplary one of such display apparatus is disclosed in Japanese Utility Model Laid-Open No. 14126/1986 and shown in FIG. 11.
Referring to FIG. 11, the display apparatus shown includes a half mirror 30 disposed on an inner face of a windshield 10 of a vehicle at a location below a rear edge of a front hood 20, and a display unit 50 having a display face directed toward the windshield 10 such that, when viewed from a visual point position above a driver's seat of the vehicle, a display image on the display unit 50 may be reflected on and visually observed by way of the half mirror 30. With the display apparatus, the display image appears in the proximity of a field of view of the windshield 10, and accordingly, a so-called head-up display function is obtained wherein movement of a line of sight of a driver is reduced. Further, a ceramic coating 60 is provided in the rear of the half mirror 30, that is, on an outer face of the windshield 10, so that a display image formed on the display unit 50 and reflected by the half mirror 30 may be visually observed on the dark background provided by the ceramic coating 60.
With a display apparatus of the type described just above, however, since the display face of the display unit is directed uprightly, external light such as, for example, light of the sun at a high location or light of a street lamp is sometimes admitted directly to the display face of the display unit through the windshield, which will cause the following problems.
In particular, a display apparatus which makes use of a reflecting face as described above commonly employs a display unit of the self light emitting type such as a fluorescent character display tube or a liquid crystal display unit with a back light. With a display unit of the type mentioned, if intense light is irradiated upon the display face thereof, then a so-called washout phenomenon takes place wherein the brightness at a light emitting portion is reduced relative to that at a no light emitting portion to lower the contrast extremely so that such no light emitting portion may be visually observed or the display image may not be visually observed easily.
In order to eliminate the influence of external light, it seems effective to mount a smoke filter in front of the display face to interrupt intense external light and raise the brightness of light to be emitted from the display unit to raise the contrast. Such a means, however, is disadvantageous in that the output of the display unit must be increased.
Alternatively, louvers or like means are sometimes provided in order to control the incoming direction of external light and the outgoing direction of display light. With such means, angular ranges within which display light and external light passes through the louvers can be set by suitably setting the height and the pitch of the lourvers. However, in order to prevent deterioration of the quality of a display image, the pitch of the louvers must be made small, which makes production of the louvers difficult. Further, since the transmission factor of light through the louvers is rather reduced, the output of the display unit must be raised as much.